1. Field of the Invention
The invention relates to a process for distillative separation of a given mixture of three or more components into its constituent parts and also to an apparatus for carrying out the process by suitable use and coupling of two or more distillation columns.
2. Description of the Related Art
Distillative processes are commonly used in chemical engineering in order to thermally separate mixtures of different relative volatility and/or mutually soluble substances. Distillation forms part of the range of thermal separation processes.
Continuous distillative fractionation of multicomponent mixtures can be carried out using various process variants.
In the simplest case, a feed mixture consisting of one low and one high boiler fraction is fractionated into those two constituent parts. The mixture to be separated is introduced between the top and the bottom of the distillation column. The feed inlet divides the column into a stripping section and a rectifying section.
The high-boiler fraction is withdrawn in the bottom of the column. Some of this fraction is evaporated and fed back to the column, for example by natural circulation, to heat the column. The low-boiler fraction exits at the top of the column as vapor and is liquefied in a condenser. Some of this condensate is recycled back into the column and flows downward as reflux in countercurrent to the ascending vapors.
However, in the separation of feed mixtures consisting of a multicomponent mixture into more than two fractions, two or more conventional distillation columns then need to be used. In a simple case, this requires N−1 distillation columns for an N-component mixture.
FIG. 1 shows the a/c path for separation of a three-component mixture ABC comprising low boiler A, middle boiler B and high boiler C.
It is preferable to use the a/c path when the proportion of middle boiler in the feed is at its greatest. The a/c path represents an energetic optimum for this case. The separation in the first column ensues such that the top product contains no high boiler C and the bottom product contains no low boiler A. The middle boiler B is present in both the top fraction and the bottom fraction of the first column. Each of fractions AB and BC is fractionated in a downstream column into the pure products A, B, C. This variant therefore requires three separating steps.
An alternative to coupling two or more distillation columns is provided by dividing wall columns, i.e. columns that prevent transverse mixing of liquid and vapor streams in sections of the column by means of a vertical dividing wall disposed in the longitudinal direction of the column. This column thus comprises a vertical dividing wall which runs along part of the column height and divides the cross section into two segments to the left and right of the dividing wall.
FIG. 3 shows a conventional dividing wall column in which high boiler is led off as bottoms, the medium boiler via the side draw, and the low boiler via the top stream.
In this case it is possible to fractionate, for example, a three component mixture into its three pure component parts, for which two conventional columns would normally be required.
The dividing wall 5 disposed in the longitudinal direction of the column divides the column interior into a feed section 1, a withdrawal section 2, an upper combined rectifying section 33 and a lower combined stripping section 43 (black).
The feed inlet of the mixture to be separated is generally positioned in a central region between an upper and a lower region of the feed section 1. It is also possible to provide one or more further feed inlets between an upper and a lower region of the feed section 1.
In the withdrawal section 2—to the right of the dividing wall—one or more side draws are disposed between an upper and a lower region. It is also possible to provide a further side draw between the lower and a lowest region of the withdrawal section 2.
WO 2009092682 A1 discloses a process for the distillative work-up of 1,5,9-cyclododecatriene (CDT) and an apparatus for carrying out the process. The achievement of the object starts from a process for distillative work-up of crude CDT obtained by trimerization of butadiene.
Dividing wall columns are used to distillatively separate the crude CDT produced as a multicomponent mixture. The dividing wall, which can consist of one plate or of two or more joined individual plates, longitudinally divides the central section of the column into a feed section and a withdrawal section. With regard to separatory internals that can be used in the dividing wall column, both random packings and structured packings or separating trays are useful. It is possible to form the dividing wall from loosely inserted subsegments.
U.S. Pat. No. 6,884,324B2 discloses a column for concentrating phthalic anhydride (PA), this column having two distillation stages, wherein the distillative removal of the low boilers in the crude PA is effected in the first distillation stage and the removal of the high boilers from the pure PA is conducted in the second distillation stage, the two distillation stages being disposed side by side and being completely separated from one another by a vertical wall, wherein the bottom of the first distillation stage is connected to the bottom of the second distillation stage. The bottom of the first distillation stage can be connected to the bottom of the second distillation stage by an overflow pipe. Equally, the bottom of the first distillation stage can be connected to the bottom of the second distillation stage by a pump.
The prior art thus discloses using two or more distillation columns performing different separation functions in an integrated system, or a dividing wall column, to separate multicomponent mixtures.
Using the dividing wall columns described, total savings of approximately 30% can be realized, based on running costs and capital costs, compared to a conventional series connection of two columns. Dividing wall columns are therefore preferable compared to the conventional interconnection of distillation columns.
However, dividing wall columns generally need to have larger dimensions than the corresponding individual apparatuses they are to replace. The construction height of the dividing wall column generally equates to at least the construction height of one of the individual apparatuses and to no more than the sum of the construction heights of the individual apparatuses.
The diameter of the dividing wall column equates, as a function of the hydraulic loading, to at least the smallest diameter of the individual apparatuses and to no more than the greater diameter of the individual apparatuses.
Depending on the separating task (multicomponent mixture), the consequence may be extreme construction heights, high column diameters and therefore high capital costs, which is disadvantageous.
These problems give rise to the objective of the invention.